Effective g-tensor for the user-specified Kramers doublet, computed as described in http://dx.doi.org/10.1063/1.4793736
Lanthanide systems often have multiple pairs of energy levels that are related to one another by a spin flip. These are called Kramers doublets. This function returns the effective g-tensor for the user-specified pair of levels.
states - the numbers of the states to use (numbered sequentially from the lowest to the highest energy)
g - 3x3 g-tensor matrix in Bohr mag- neton units
See dy_lft_single.m file in examples/giant_spin folder of Spinach distribution.
Make sure that the pair of levels specified actually is a Kramers doublet.